The invention relates to a method and apparatus for treating lime mud, and a particular configuration of a rotary lime kiln at a pulp mill. A lime kiln is part of the pulp mill chemical recovery plant, the lime being used for causticizing green liquor to produce white liquor in the production of kraft pulp.
Lime mud is produced in the causticizing plant of a sulphate pulp mill. For reuse the lime mud which is mainly calcium carbonate (CaCO3) is regenerated by reburning it to form calcium oxide (CaO). The reburning takes place in an ordinary rotary kiln, into the upper end of which the lime mud is supplied. The mud flows slowly downwardly through drying, heating and reaction zones. Conventional rotary kilns desirably enhance the heat transfer from the flue gases and the lime mud in the drying zone by providing chains and/or lifters, which rotate with the kiln and come into contact with the lime mud during that rotation. In order to obtain proper results the heat treatment of lime mud typically takes place slowly in the kiln, meaning that the kiln must be long and therefore takes up significant floor space or land area.
One prior art method to allow a shorter kiln is a suspension-type drying system. In such systems lime mud is introduced into a vertical conduit through which the flue gas from the kiln moves upwardly at a relatively high rate of speed. Most of the lime mud is entrained in the upwardly moving gas, and dried by contact therewith, and the flue gas with entrained particles is fed to a conventional separator (such as a cyclonic separator), the flue gas being discharged and the lime mud particles—which now have been dried—being fed to the inlet to the lime kiln. U.S. Pat. No. 5,110,567 discloses thickening of lime mud in a lime filter to a dryness of over 75%, whereby it is possible to introduce the lime mud directly into a suspension dryer without the necessity of using hammer mills or mixing screws for crushing lumps of lime mud. The lime mud is so dry that the dryer will remain open. Another point of feeding lime mud is, however, still needed for the lime reburning kiln. When the “precoat” of the lime mud filter is replaced, it is not desirable to feed the moist lump of lime mud into the suspension dryer because there is a risk of the dryer becoming clogged, but the lime mud lump is introduced directly into the lime reburning kiln by means of a separate feed screw.
U.S. Pat. No. 5,413,635 discloses a method, in which a controllable amount of flue gas that has exited a flash dryer and has been separated from the dried lime mud is recycled back into the vertical portion of the flash dryer below the feed inlet portion of the flash dryer. The aim is that the velocity of the gas through the flash dryer is maintained at a level sufficient to entrain all of the lime mud feed in the gas stream. In a process malfunction, where the gas velocity is sufficient, the moist mud falls down. This may cause plugging.
U.S. Pat. No. 5,213,496 discloses a method for feeding lime mud to a lime kiln according to which method all the moist lime mud from a lime mud filter is supplied to the upper end of a feed chamber of the kiln. Lime mud may be transported from this chamber either to a suspension dryer or directly to a kiln or both, depending on the dry solids content and particle size of the lime mud. There is a partition wall in the upper portion of the feed chamber dividing the chamber into two flow channels. The amount of lime mud entering the dryer may be regulated by changing, by means of a control baffle disposed in the upper section of the partition wall, the relationship between the gas flow volumes flowing through the adjacent flow channels. In the lower end of the flue gas chamber, there is a spiral feeder, which transports the moist lime mud falling into the lower end to the kiln. Dried lime mud from the separation apparatus is brought via a return duct to the vicinity of this spiral feeder. Such systems—namely combinations of suspension-type dryers and rotary kilns—are replacing rotary kilns per se in the marketplace because the combination of a suspension-type dryer and rotary kiln provides high heat capacity and good heat economy. Capacity and heat economy of such a combination may be further improved by employing two subsequent suspension drying stages, one of which serves as a dryer per se, the other acting as a preheater. The flue gases from a kiln are first taken to a suspension preheater and from there to the dryer. The lime mud to be dried is supplied from a lime mud filter to the dryer, then to the preheater, and finally to the kiln to be calcinated therein.
In U.S. Pat. No. 5,711,802 (European patent No. 751916) it is stated that a disadvantage of the plant having the above-described drying and preheating stages is that the temperature of the preheating stage may become so high that lime mud tends to stick onto metal surfaces. It is further stated that dry sticking problems occur typically at temperatures in the range of 400-600° C., depending on the dry solids content of the lime mud. In U.S. Pat. No. 5,711,802, the sticking of lime mud is prevented so that the temperature of the preheating stage does not exceed a temperature between 400-600° C. The temperature is regulated either by feeding part of the moist lime mud directly into the preheating stage or by directing part of the flue gas from the lime kiln directly into lime mud drying, thus bypassing the lime mud preheating stage. However, both regulation methods are complicated to accomplish in practice. The disadvantage of the first-mentioned method is the feeding of moist lime mud into two separate locations positioned far from each other. This results in both a complicated transporting and feeding apparatus and increasing maintenance load. In the latter regulation method, the problem consists in the large amount of flue gas transportation piping and the fact that maintaining an adequate flue gas flow in the piping may in extreme situations (i.e. very moist or very dry lime mud) require special arrangements.
When feeding dried and/or preheated lime mud into the kiln, metallic constructions of the feed end of the kiln are exposed to severe stress in the gas exit temperatures. The strength of metals begins to decrease at high temperatures, even though their heat resistance otherwise would stay at a reasonable level. Further, at these temperatures lime mud tends to stick onto surfaces. FI patent No. 106642 discloses a method, according to which part of the thickened moist lime mud is dried and preheated by means of flue gas from the lime kiln, separated from the flue gas and fed into the feed end of the lime kiln. For cooling the constructions of the feed end of the kiln, part of the moist lime mud is fed directly into the feed chamber of the lime kiln, bypassing the flue gas treatment, in order to cool the feed end constructions.
FI patent No. 108235 discloses a method, according to which moist lime mud is dried by means of flue gas originating from lime mud calcination and separated from the flue gas, and the dried lime mud is preheated by means of flue gas originating from lime mud calcination, separated from the flue gas and fed into a calcination apparatus. The temperature of the preheating is regulated into a certain value in the range of 400-600° C. by circulating part of the preheated lime mud into the lime mud drying stage.
Even though it may be stated based on the above that several solutions have been presented for regulating the temperature in lime mud treatment with flue gases and at the feed end of the lime kiln, there still exists a need to find a simpler method for controlling the temperature in connection with lime mud feeding, so that e.g. lime mud sticking can be decreased or eliminated. Further, the known kilns include a feed chamber/smoke chamber connected to the kiln shell which makes the kiln longer.